Energy Revision set 1

An electric motor provides 900 J of useful output energy. The efficiency of the motor is 60 %. How much energy is transferred by electricity to the motor?

• 15 J

• 540 J

• 1500 J

• 5400 J

Answer explanation

e

A horizontal force pulls a box along a horizontal surface. The box gains 30 J of kinetic energy and 10 J of thermal energy is produced by the friction between the box and the surface. How much work is done by the force?

• 10 J

• 20 J

• 30 J

• 40 J

Answer explanation

Conservation of energy

work done = energy transferred = kinetic + thermal

A child pushes a toy car along a horizontal surface and then releases it. As the car slows down, what is the main energy transfer?

• from chemical to thermal

• from chemical to kinetic

• from kinetic to gravitational (potential)

• from kinetic to thermal

Answer explanation

Kinetic energy store of car transferred to thermal energy stores (car/brakes/surroundings)

In a small hydroelectric power scheme, 800 kg of water drops through a vertical height of 2.2 m every second. The electrical output is 10.6 kW.

What is the efficiency of the scheme?

• 0.60%

• 17%

• 60%

• 75%

Answer explanation

Change in GPE of water/second = mgh

= 800 kg x 10 N/kg x 2.2 m = 17600 J/s (W)

Efficiency = (10 600 W ÷ 17600 W) x 100%

The diagram shows a circuit.

Which energy transfers occur (location of energy stores)?

• battery → lamp → surrounding air

• battery → surrounding air → battery

• lamp → surrounding air → battery

• surrounding air → lamp → battery

Answer explanation

Chemical energy in battery transferred by electricity to lamp the thermal store of surroundings

The equation used to find the change in gravitational potential energy of an object can be written as

∆E_P = Y × Z × ∆h

where ∆E_P is the change in gravitational energy and ∆h is the change in height. Which box gives the quantities Y and Z?

• Y - mass

  Z - gravitational field strength

• Y - gravitational field strength

  Z - density

• Y - density

  Z - speed

• Y - mass

  Z = speed

A ball is falling towards the ground. Which transfer between energy stores is taking place?

• elastic energy to kinetic energy

• gravitational potential energy to elastic energy

• gravitational potential energy to kinetic energy

• kinetic energy to gravitational potential energy

Answer explanation

The diagram shows the energy transferred in a lamp in one second.

Which store of wasted energy is transferred by the lamp?

• chemical potential energy

• electrical energy

• gravitational potential energy

• thermal energy

Answer explanation

Final energy store is always thermal

An electrical heater transfers thermal energy to the surroundings. The graph shows how the amount of thermal energy transferred varies with time.

The heater continues to transfer energy at the same rate. How much thermal energy is transferred by the heater in 5.0 minutes?

• 200 J

• 400 J

• 2 000 J

• 12 000 J

Answer explanation

Rate of energy transfer = 40 J/s (400J÷10s)

40 J/s x 300s = 12 000 J

In an experiment, an object is heated. The data from the experiment is shown.

● The energy transferred to the object is 3.0 kJ.

● The rise in temperature of the object is 10 °C.

● The specific heat capacity of the object is 150 J / (kg °C).

What is the mass of the object?

• 1 kg

• 2 kg

• 2 g

• 4500 kg

Answer explanation

m = ΔE ÷ (c x Δθ) = 3000/(150x10)